1. Field of the Invention
The invention relates to a device for mixing the components of a mass flow flowing through it, in which the components may, in particular, be in solid, liquid or gas form by means of a hydrodynamic supercavitation field, in order to generate a mixture, in particular an emulsion or a suspension.
2. Description of Related Art
If what is known as the static pressure in a liquid flowing in, as a result of a flow constriction, locally falls below the vapor pressure, cavitation occurs, i.e. vapor-filled gas bubbles, which are also known as cavitation bubbles, are formed in the liquid. If the static pressure then increases again and exceeds the vapor pressure, these gas bubbles collapse implosively (practically at the speed of sound).
This mechanism of hydrodynamically generated cavitation is covered by the Bernoulli-equation. According to this equation, it is generally the case (cf. “Gerthsen Physik”, Helmut Vogel, ISBN 3-540-59278-4, 18th Edition, Springer-Verlag Berlin Heidelberg New York, 1995, Chapter 3.3.6, Strömung idealer Flüssigkeiten [Flow of ideal liquids], pp. 118 to 121) on every potential surface of the external volumetric forces in a filament of flow which flows in, i.e. everywhere at the same height in the case of the force of gravity, thatp+½ρv2=p0=const,where p0 is the pressure which would prevail in the stationary liquid, for example air pressure plus the hydrostatic pressure ρgh. The sum of the static pressure p and the dynamic pressure ½ρv2 has the same value everywhere at a given depth.
When the flow velocity reaches or exceeds the value vk=√2ρ0/ρ, the static pressure becomes zero or negative. Such velocities (in water vk=14 m/s) are easily reached in all high-speed water-craft, in low-speed water-craft at least at the propellers and also at turbine blades and in liquid pumps. Even slightly beforehand, the static pressure falls below the vapor pressure of the liquid, which is a few hundred Pa, and cavitation occurs, in particular, if microscopic air bubbles are already present as nuclei, which is difficult to avoid.
Therefore, the phenomenon of hydrodynamic cavitation consists in the formation of hollow spaces which are filled with a vapor gas mixture, known as the cavitation bubbles, in the interior of a fast-flowing liquid flow or at peripheral regions of a body which it is difficult for medium to flow around and which is arranged in the flowing liquid flow, in each case as a result of a local pressure drop caused by the liquid movement (flow). Therefore, hydrodynamic cavitation occurs in all hydraulic systems in which considerable pressure differences occur, such as turbines, pumps and high-pressure nozzles.
In the case of ultrasonic cavitation, in the sub-atmospheric pressure phase of a sound field the tearing stresses of the material are exceeded, so that once again the cavitation bubbles filled with vapor or gas are formed. In sonochemisrtry, the extreme conditions which occur on collapse (pressure, temperature) of the cavitational bubbles generated in the ultrasound field are exploited. The physical effect of sonoluminescence is also associated with the dynamics of cavitation bubbles and their generation by means of an ultrasound field.
The examples mentioned above relate to cavitation which occurs in the flow field or in the acoustic field as a result of a tensile stress which is present in the water or a liquid. Generating a further type of cavitation involves locally depositing energy in the liquid, for example by means of a spark or a laser pulse. Details of the latter are to be found, for example in the thesis written by Olgert Lindau, “Dynamik und Lumineszenz lasererzeugter Kavitationsblasen”, [Dynamics and luminescence of laser-generated cavitation bubbles], 1998, written at the Third Physics Institute of the Georg-August-Universität in Göttingen.
It is known that cavitation and the associated effects can be used to mix the components of a flowing mass flow. Therefore, by way of example, two different liquids or a liquid and a solid (particles) or a liquid and a gas can be mixed with one another. The mixing, emulsifying and dispersing action of the cavitation is based on the action of a large number of forces originating from collapsing cavitation bubbles on the mixture of components which is to be treated. The implosion of cavitation bubbles in the vicinity of the interface between two solid-liquid phase regions is accompanied by the dispersion of the solid phase (particles) in the liquid phase (liquid) and by the formation of a suspension. Similarly, the implosion of cavitation bubbles in the vicinity of the interface between two different liquid phases is accompanied by one liquid being broken up in the other and the formation of an emulsion. In both cases, the interface between the continuous phases is destroyed, i.e. eroded, and a dispersion medium and a disperse phase are formed.
U.S. Pat. No. 3,834,982 has described a device for generating a suspension of fiber materials. The device comprises a housing having an entry opening for supplying components of a fiber-material suspension and an exit opening for removing the fiber-material suspension produced by cavitation, and a through-flow chamber with a cylindrical body, which comprises a single piece and is difficult for medium to flow around (and which is generally also known as a cavitator on account of its function), placed therein. The component flow flows through the through-flow chamber and the cylindrical body, which it is difficult for medium to flow around, positioned therein, which body is arranged transversely with respect to the direction of flow, so that it generates local narrowing of the fiber-material suspension. Therefore, a hydrodynamic cavitation field is formed behind the cylinder, i.e. the cylinder generates a three-dimensional region in the flowing mass flow in which, in a dynamic process, cavitation bubbles are formed, are present and collapse (implode).
On account of the shape of the one cylindrical body which it is difficult for medium to flow around in U.S. Pat. No. 3,834,982, only a single cavitation field is formed behind this body as a result of the cross-sectional narrowing of the flow cross section which it produces. Therefore, this device effects only relatively poor mixing of the components of the fiber-material suspension with regard to the homogeneity (particle size) and long-term stability of the dispersion produced. The intensity of the cavitation field produced using the device described in U.S. Pat. No. 3,834,982 is too low for mixing or dispersing phases which are difficult to mix or disperse.
The cavitation mixer described in SU-A 1088782) additionally has a means which allows further pressure oscillations generated by means of a compressed-air source to be superimposed on the cavitation field.
The cavitation mixer disclosed in SU-A 1678426 has an axially elastically mounted body which it is difficult for medium to flow around and which is intended to cause its own resonant vibrations in the liquid medium.
SU-A 1720695 has described a further cavitation mixer which, as the body which it is difficult for medium to flow around, has two hemispheres which between them delimit a rectangular groove. The pulsation of the flow in the groove is intended to act on the cavitation region and in this way to increase the frequency of cavitation bubbles and their intensity.
Therefore, the three documents cited above disclose cavitation mixers in which the mixing effect is to be improved by attempting to improve the cavitation action by means of further separation edges or by superimposing pressure waves which correspond to further separation edges.
DE-A-3610744 has described a device for the direct aeration and recirculation in particular of waste waters, which uses an impeller to generate a cavitation field and mixes air into water.
U.S. Pat. No. 4,127,332 has disclosed a further mixing device which uses cavitation for this purpose.
Compared to the cavitation mixes described above, in which in each case only one cavitation field is generated, in order to mix two different components of a system, the cavitation effect and therefore the mixing effect is significantly improved in cavitation mixers which generate what is known as a super-cavitation field, i.e. one which superimposes a plurality of cavitation fields.
For example, DE-A 4433744 has disclosed a cavitation mixer which, as the body which it is difficult for medium to flow around (cavitator), has a truncated cone which is formed from a plurality of partial bodies which it is difficult for medium to flow around and between each which there is a hollow space through which medium can flow. This body around which it is difficult for medium to flow is arranged in a fixed position in a passage chamber which—as seen in the direction of flow—has a constant circular cross section throughout the whole of the body which it is difficult to flow around.
A first cavitation field is generated in a customary way as a result of medium flowing around the entire body. Furthermore, the hollow spaces through which medium can flow form a further source for cavitation fields which are formed by the flow in these hollow spaces, which in particular are also directed upwardly into the flows flowing around the body as a whole, so that the cavitation bubbles in the hollow spaces through which medium can flow also merge outward into the conventional cavitation field. The three-dimensional superimposition of the individual cavitation fields generates what is known as a supercavitation field and results in multiplication of the cavitation effect of each individual cavitation field.
Hydrodynamic supercavitation generators as in DE-A 4433744 represent effective mixing devices which can be used to process, for example, mix, emulsify, homogenize, disperse or dissolve, a flowing fluid comprising a plurality of components or to saturate liquids with gases. Supercavitation generators are universal devices for processing a wide range of products in the chemical, petrochemical, cosmetic and pharmaceutical industries and also in the ceramics and foodstuffs industries and in other branches of the economy.
Typical basic technical data for a hydrodynamic supercavitation generator and parameters of the medium to be processed are:
Productivity:0.1 to 500 m3/hAdmission pressure:0.3 to 1.2 MPaSubstance viscosity:0.001 to 30 Pa · sSubstance temperature:5 to 250° C.Overall length:50 to 800 mmDiameter of the working chamber:15 to 300 mmMass:0.4 to 40 kgMinimum duration of use:30 000 h
The mixing and homogenization processes in the mixer are based on the use of the hydrodynamic cavitation and are linked with physical effects such as pressure waves, cumulation, self-induced vibrations, vibration turbulence and parallel diffusion, by way of example, which occur when cavitation bubbles collapse. The volumetric concentration of the cavitation bubbles in the equipment reaches orders of magnitude of 1 to 1010 1/m3. When each cavitation bubble collapses, pressure pulses are initiated, which reach 103 MPa and above, and, as in the implosion of a cavitation bubble, temperatures of around 5000 K occur in the bubble (cf. for example VDI Nachrichten, Apr. 1, 1999, No. 13, “Schadstoffe im Ultraschall” [Harmful substances in ultrasound]). At the high volumetric concentration of the bubbles in the working range of the mixer, such high pressure pulses contribute to the pulsed power fed to a volumetric unit of the medium which is to be processed amounting to 104 to 105 kW/m3. It should also be noted that a vacuum zone with a pressure of 4 to 10 kPa is generated in the working chamber of the mixer, making it possible for various liquid and gaseous components to be injected directly into the mixer.
EP-A 0 644 271 has likewise disclosed a hydrodynamic supercavitation mixer which includes a body which it is difficult for medium to flow around and which comprises at least two elements which ensure the formation of their own cavitation fields. The elements or partial bodies which form the body which it is difficult for medium to flow around may be in the form of hollow truncated cones or hemispheres and moreover may each be secured to a hollow bar. These bars are designed in such a way that they can be fitted into one another and can each be connected to individual devices, so that they can be displaced in the axial direction with respect to one another. In this way, the individual elements which form the body which it is difficult for medium to flow around can be axially displaced with respect to one another in the direction of flow and in this way can be arranged at different distances in relation to one another. In this way, it is possible to vary and adjust not only the shape of the elements but also by means of the distance between the elements, the properties of the hydrodynamic cavitation field produced by each element, which in turn has a corresponding effect on the superimposition of the individual cavitation fields, i.e. the supercavitation field of the cavitation mixer.
EP-A 0 644 271 also teaches that to optimize the processes of dispersion and emulsification it is expedient for a gaseous component to be introduced into the hydrodynamic flow of components at least in a section of its local constriction, or immediately downstream thereof. The elements of the body which it is difficult for medium to flow around may also consist of an elastic, nonmetallic material. Moreover, the cavitation mixer may include a further, additional body which it is difficult for medium to flow around, which, as seen in the direction of flow, is arranged downstream of the first body which it is difficult for medium to flow around and which it resembles, and which is connected to this first body which it is difficult for medium to flow around by an elastic element, in such a manner that it can be displaced along the axis of the through-flow passage.
In addition to the adjustable element of the body which it is difficult for medium to flow around, the process or device described in EP-A 0 644 271 also offers the possibility of regulating the intensity of the hydrodynamic supercavitation field which is formed to match the specific technological process sequences. However, the body which it is difficult for medium to flow around as a whole is arranged at a fixed location in a through-flow passage which, moreover, has a constant circular cross section in the region of the body which it is difficult for medium to flow around and as seen in the direction of flow.
Although the hydrodynamic supercavitation generators according to the prior art generally provide good results, there is nevertheless a need for improvement in many respects.